1. Field of the Invention.
The present invention relates generally to the defect testing of non-ferromagnetic materials, and, more particularly, to such testing of semifinished materials.
2. Description of Related Art.
It is known to determine the presence of defects in metal objects by inducing eddy currents in the objects and examining the field produced thereby. Such techniques have been used to test a variety of semifinished materials, such as wire, rods and pipes. In particular, the tests are made after the test samples have undergone a shaping or fashioning, e.g. in a rolling mill or the like, and in this way you can detect cracks, shrink holes, laps, shells and other defects, to the extent they are located immediately under the outer surface of the test sample.
Where ferromagnetic materials are found to be present in a non-ferromagnetic object this indicates defects in the shaping tools, e.g. wearing of rollers and splitting-off of particles which have then been rolled into the semifinished material. On the other hand, inclusions of ferromagnetic material in a test object may give rise to serious results on further processing of the semifinished material, such as splittings, breaks or the like, damaging the drawing tool when drawing semifinished material to smaller and smaller diameters.
The utilization of past known eddy-current methods for the detection of ferromagnetic inclusions in an otherwise non-ferromagnetic body has not been totally satisfactory. First, so-called "skin effect" prevents eddy currents from penetrating deep into the material and even inclusions disposed only slightly underneath the surface are not indicated. It is true that the penetration depth may be slightly improved by lowering test frequency. This is achieved, however, at the cost of the recognition of surface defects and reduces the potential test speed. A second deficiency is based on the fact that, beside the inclusions extending up to the surface, further categories of surface defects are detected, too, by means of the eddy-current test. The relation of the importance of a defect to the respective signal voltage is, however, very different, depending on whether inclusions or other defects are concerned. A definite evaluation of the importance of defects has not been possible in the past, if different categories of defects occur to the same extent.
Another technique is known by which the detection of inclusions of ferromagnetic material in metal semifinished material is possible. For example, if the semifinished material is passed through a coil, in the region of which a strong, constant magnetic field is built up, the ferromagnetic particles passing through the coil act as magnetic dipoles and induce an electrical voltage in the coil. EQU v=n.multidot.d.phi./dt, (1)
with n being the number of windings of the coil and .phi. the magnetic flux penetrating the coil which results from the dipole. Accordingly, the presence of inclusions of ferromagnetic material in metal semifinished material can be detected, however not in the presence of any other surface defects, although the latter might easily be detected by the eddy-current method.